The glass disclosed in Patent Document 1 is known among optical glasses for preparing glass optical elements such as aspherical lenses by precision press molding to be glass having a B2O3—La2O3 composition. The glass disclosed in Patent Document 1 is high refractive index, low dispersion glass with low dispersion even for high refractive index glass. Depending on the application of the optical element, high refractive index glass exhibiting higher dispersion characteristics than the glass described in Document 1 (WO2009/144947A1) or English language family members US2010/255979A1, U.S. Pat. No. 8,338,320, and US2013/079213A1 is required. The glasses disclosed in Document 2 (Japanese Unexamined Patent Publication (KOKAI) No. 2005-247613) or English language family members US2008/167172A1, U.S. Pat. No. 7,622,409, US2010/035744A1, U.S. Pat. No. 7,932,197, and US2005/197243A1; Document 3 (Japanese Unexamined Patent Publication (KOKAI) No. 2011-93781) or English language family member US2011/105294A1; Document 4 (Japanese Unexamined Patent Publication (KOKAI) No. 2011-225384) or English language family member US2011/257001A1; Document 5 (Japanese Unexamined Patent Publication (KOKAI) No. 2011-225383) or English language family member US2011/257002A1; Document 6 (Japanese Unexamined Patent Publication (KOKAI) No. 2010-30879, US2009/325779A1) or English language family members U.S. Pat. No. 8,110,514, US2012/100981A1, and U.S. Pat. No. 8,410,008, which are expressly incorporated herein by reference in their entirety, are known examples of such glasses.
The glasses described in Documents 2 and 4 to 6 are glasses that can be precision press molded. However, when a single pressing mold is used to repeatedly precision press mold several hundred articles using one of these glasses, clouding will be sometimes generated on the surface of the glass article obtained or the glass will sometimes fuse to the mold. Accordingly, an improvement in the glass molding property is desirable for large quantity production. This is thought to be occurring because the ratio of high refractive index, high dispersion components is increasing as the glass dispersion is rising.
Further, in the course of preparing a glass material for press molding, crystals sometimes precipitate out onto the surface of the glass when using the glass described in Document 3. Accordingly, an improvement in the thermal stability of the glass is desirable for large quantity production of glass materials.
As described above, increasing the suitability to precision press molding requires increasing the thermal stability during manufacturing of the glass.
An aspect of the present invention provides for optical glass having high refractive index and high dispersion characteristics that is excellent in thermal stability during glass manufacturing in addition to imparting the good press-molding property; to a precision press molding preform and an optical element, each of which is prepared from the above glass.